DT has basically given up in T-mo, so if the merger doesn't happen, T-mo will probably shut down in a couple years, or be bought for significantly cheaper than it is now. I bet that would be real good for the people who would otherwise be able to keep their jobs...
AT&T could use the spectrum and service area to build out. And, T-Mo customers would gain a significant increase in service area and quality as well. Seriously, try to use T-Mo where I live, and IF you get signal, good luck keeping it. Don't even get me started on Verizon.
The DOJ let Verizon buy up properties, including Alltel, with no real gripes. But, as soon as AT&T does anything, the DOJ is all over them.
A monopoly is one company controlling enough of a certain market to essentially be the only option, with no competition. Even if the merger goes through, this is NOT going to be the case. There is still Sprint, and VZW on a national level, and several small regional carriers everywhere else.
Someone has bad blood with AT&T and for whatever reason, they have become a DOJ target.